Experimental factors relevant to approaches to gene therapy for cystic fibrosis are being studied. The effects of various experimental parameters related to (e.g., specific ligand, polylysine linker chain length, ratio of ligand to lysine, degree of substitution) on outcome are being evaluated in terms of in vitro experience over time. These results will assist in the design of future studies, and it is hoped that these will ultimately inform the design of in vivo approaches. A number of gene transfer systems have been developed to specifically deliver foreign DNA via the receptor mediated endocytosis pathway. A carrier, usually consisting of a polycationic protein chemically conjugated to a receptor-specific ligand, is used to condense DNA and present it to the target cell. Manipulation of ionic strength can produce complexes containing a single plasmid DNA molecule. These efforts have targeted the serpin enzyme complex receptor for gene transfer. Based upon results of initial studies, conjugates were constructed containing one ligand per polymer; these conjugates were then systematically diluted with free polymer, and those mixtures assayed to determine the optimal number of ligands per DNA molecule required for efficacy. It was found that the extent of substitution with ligand and chain length of polyK contained in receptor-mediated gene transfer complexes affect both the intensity and duration of transgene expression. As few as 5-6 ligands per plasmid DNA molecule can effectively direct DNA delivery by receptor-mediated means. These observations may permit tailoring of complex construction to the usage required--short duration, high intensity expression, or activity which is slower to peak but more persistent.